This invention relates to phased array antenna systems and in particular to a technique for reducing the number of phase shifters or other active components in a phased array which must radiate within only a limited region of space.
Conventional phased array antenna systems are well known and usually have a phase control unit associated with each of the radiating elements. Phase control units require electronic components and are very often the most expensive part of a phased array system. When a conventional phased array having a phase control device associated with each element of the array is required to scan only a limited portion of real space, that is less than plus or minus 90.degree. from broadside, such an array has more scan capability than required, and the large number of phase control units results in a high system cost.
A phased array system should ideally have approximately one active control unit, for example, a phase shifter or switch, for each beam width it is required to scan. There are prior art systems for scanning an antenna beam over a limited region of space using approximately one control unit for each beam width. These systems usually utilize switching techniques to select the desired beam. For example, the well-known Butler Matrix may be used in conjunction with a switching circuit and an array of elements, so that by switching the source of wave energy signals to the various inputs of the Butler Matrix, the antenna beam is switched to various beam positions. A similar result may be achieved by optically illuminating a focusing device from a variety of feed locations. One such technique is described in U.S. Pat. No. 3,881,178, Peter W. Hannan, entitled "Antenna System for Radiating Multiple Planar Beams", which is assigned to the same assignee as the present invention.
In U.S. Pat. No. 3,803,625, entitled "Network approach for Reducing the Number of Phase Shifters in a Limited Scan Phased Array", Nemit describes a technique for reducing the number of phase shifters required in a limited scan array. Nemit's technique involves the use of overlapping sub-arrays of antenna elements each of which is associated with a phase shifter. Each sub-array has a pattern which suppresses the amplitudes of grating lobes in real space, thereby enabling a larger spacing between sub-arrays than would be allowable in a conventional array wherein each sub-array is a single element. In his patent, Nemit describes a condition which may achieve an ideal sub-array pattern and discloses the criteria for arriving at the minimum necessary number of phase control units. Nemit does not, however, describe a practical technique for achieving the ideal sub-array pattern.
The technique described by Nemit involves the direct physical interconnecting of each sub-array input port with all of the antenna elements to be excited by wave energy signals supplied to that input port. This approach cannot be practically implemented to achieve a near ideal sub-array radiation pattern, because it requires an excessive number of individual interconnecting transmission lines, particularly in an actual array which has a large number of radiating elements.